Predator-prey cycles

In a healthy, balanced ecosystem the numbers of predators and prey remain fairly constant. They can go up and down during each year, but generally over the years, these increases and decreases remain fairly constant.

If numbers of either predators or prey increase or decrease, it could be due to a change in the abiotic factors, like water or sunlight, or biotic factors, like a new predator or pathogen. This would result in a less balanced ecosystem.

An example is shown in the graph below and shows characteristic repeating patterns called predator-prey cycles.

Predator prey graph.  As the number of predators increases the number of prey descreases. As the number of predators decrease the number of prey increase.

The graph shows that there is almost always more prey than predators. It also shows the following patterns:

  1. The number of predators increases because there is more prey.
  2. The number of prey then reduces because there are more predators.
  3. The number of predators then reduces because there is now less prey.